This invention relates to fuel cells and, in particular, to a fuel cell matrix and a method of making the fuel cell matrix for use in Molten Carbonate Fuel Cells (“MCFCs”).
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which conducts charged ions. In order to produce sufficient power, individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
MCFCs generally operate at intermediate temperatures of from 575° C. to 650° C. using fuel containing carbon dioxide and carbon monoxide. A conventional fuel cell assembly includes a porous nickel anode and a porous lithiated nickel oxide cathode, separated by an electrolyte matrix storing carbonate electrolyte, such as mixtures of lithium carbonate/potassium carbonate (Li2CO3/K2CO3) or lithium carbonate/sodium carbonate (Li2CO3/Na2CO3). MCFCs generate power by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The anode and the cathode of MCFCs are isolated from one another by the porous ceramic matrix which is saturated with carbonate electrolyte. The matrix typically comprises a porous, unsintered lithium aluminate (LiAlO2) ceramic powder and is impregnated with carbonate electrolyte, and during operation, the matrix provides ionic conduction and gas sealing.
During MCFC operation, the matrix is subject to both mechanical and thermal stresses which may cause defects or breaks in the matrix. In order to provide effective gas sealing, the matrix must have sufficient strength, mechanical integrity and material endurance to withstand operational stresses, particularly during thermal cycles. In particular, the matrix has to be able to sufficiently accommodate volume changes associated with carbonate melting and solidification during MCFC thermal cycling, provide resistance to pressure differences across the matrix, and provide wet seal holding pressure over long periods of time. It is desired for the matrix to have sufficient porosity and sub-micron pore distribution to maintain strong capillary forces to retain carbonate electrolyte within the matrix's pores in order to prevent flooding of the electrodes and drying of the matrix. It is also desired that the matrix have slow or no pore growth over the MCFC's lifetime in order to continue to retain electrolyte therein by capillary forces.
Various methods of manufacturing a porous ceramic matrix having increased strength and improved electrolyte retention characteristics have been proposed. For example, coarse particles, such as aluminum oxide (Al2O3) particles in the size range of 10-120 μm, have been used in the matrix to improve compressive strength, crack resistance and thermal cycle capability. Moreover, additives, such as aluminum powder and/or carbonate compounds in powder or particulate form, have been used to improve strength and electrolyte retention capillary force. However, the use of aluminum particles in the matrix to improve strength leads to formation of undesired large pores and large core shell structures that reduce electrolyte storage capacity and stability. In particular, the aluminum particles contribute to formation of large pores and large core shell structures of greater than 2 to 6 μm within the matrix after reacting with molten carbonate electrolyte. Formation of such large pores and large core shell structure often occurs at the beginning of life, i.e., with the first 500 hours of operation, and during conditioning. FIGS. 1-2 show Scanning Electron Microscope (SEM) images of examples of large pores and large core shell structures in a conventional electrolyte matrix. Formation of large pores and large core shell structures reduces capillary force within the matrix and accelerates loss of electrolyte.
The effect of addition of aluminum particles and Li2CO3 on matrix stability and mechanical strength has also been investigated in Lee et al. J. Power Sources 179 (2008) 504-510. Lee et al. report that aluminum particle size affects snap strength of the matrix, with particles ranging from 20 μm to 30 μm providing higher strength compared to smaller particles sized at approximately 3 μm. However, the use of such aluminum particles results in formation of large pores and large core shell structures in the size range of from 10 μm to 50 μm when the aluminum particles and molten carbonate electrolyte react during conditioning and/or beginning operation of the MCFC.
In another investigation, Lee et al. used aluminum acetate, aluminum isopropoxide and aluminum acetylacetate as precursors to improve matrix strength. See Lee et al. J. Power Sources 101 (2002) 90-95. Aluminum acetylacetate was indicated as providing the improved matrix strength, though less strength than the combination of aluminum and Li2CO3 was obtained. However, all precursors studied in this investigation decompose to form Al2O3 at temperatures of approximately 400° C., resulting in poor sintering within the matrix and providing weak mechanical properties.